The overall goal of the proposed research is to determine the potential clinical utility of the analysis of minimal residual disease in patients with leukemia, and to understand the dynamics of the leukemic cell population during and following treatment. The leukemic clone is identified by the unique sequence of nucleotides present at the junction(s) of the variable, diversity, and joining regions of the immunoglobulin (lg) and T cell receptor (TCR) genes. Amplification of rearrangements by the polymerase chain reaction (PCR) followed by hybridization with an oligonucleotide probe containing the junctional sequences from the leukemic cell lg or Tcr gene allows the investigators to determine whether the cells from the leukemic clone are present, and the approximate percentage of the total that they represent. The rearranged genes from the patients with B precursor cell acute lymphoblastic leukemia (ALL) show a higher than expected rate of base pair changes from the germline sequence, indicating that perhaps somatic mutation is occurring prior to surface display of lg. To determine whether PCR analysis of gene rearrangements is useful in tracking residual disease and whether somatic mutation occurs in early B progenitor cells the following specific aims are proposed: 1) Test the hypothesis that the kinetics of disappearance of the leukemic clone during the first three months of therapy can be used prognostically to determine risk groups. Bone marrow samples taken during the first three months following diagnosis from patients being treated with various chemotherapeutic regimens will be analyzed for the extent of residual disease of PCR. Included will be patients with high and low risk features, and patients of different racial backgrounds, to determine if there are race-based differences in drug sensitivity. 2) Test the hypothesis that analysis of residual disease levels during maintenance therapy can identify impending relapse. Preliminary data indicate that PCR analysis of bone marrow samples can identify impending relapse, but no large-scale prospective study has been performed. Bone marrow samples will be analyzed at three month intervals for residual disease and determine if relapse can accurately be predicted. 3) Continue to examine the possibility that somatic mutation is occurring in pre-B and early B progenitor cell acute lymphoblastic leukemia. 4) Develop methods to automate quantitation of residual leukemia. Successful completion of the specific aims are expected to provide considerable new insight into the dynamics of the leukemic cell population during therapy, and will determine whether PCR determination of residual disease is a clinically useful tool upon which future clinical trials can be based.